Wilson: Studies in North American Peronosporales 197 
conidiophores are also more like those of P. Arenariae, but sug- 
gesting somewhat P. Dianthi. This last species, however, is some- 
what stouter than the others. The conidiophore of the American 
species is somewhat more branched than P. Arenariae and has 
much more slender ultimate branchlets than does the European 
species. As the varietal name is untenable for a species in the 
genus this fungus may be renamed Peronospora Silenes G. W. 
Wilson. 
Peronospora parasitica (Pers.) Fries and its 
segregates 
While it has been customary to consider any collection of Per- 
onospora on a Cruciferous host as certainly belonging to P. para- 
sitica a very wide range of variation has been allowed in the 
characterization of the species. True, various names have been 
applied, especially by the earlier authors, to the fungus as it 
appears on various hosts. The majority of these names, how- 
ever, represent what may be termed “ host species,” i. e. t their 
chief distinguishing characteristic is their host. 
The first valid segregation by an European mycologist was 
based on an error in the determination of the host. The case in 
point is P. Niessleana A. Berlese, based on a specimen in the 
herbarium of Niessl which was labeled P. Phyteumis Fuckel, on 
Phytcuma, but evidently not that species. So thoroughly con- 
vinced does this author appear to have been that the fungus in 
question was distinct from other recognized species that when it 
was found that the host was in reality Alliaria he retained the 
form as a subspecies under P. parasitica. What appears to be the 
same species of fungus was figured by Sowerby as Mncor Ery- 
simi. Berlese’s first judgment was better than his last, as the 
form is certainly entitled to specific rank. 
A careful study of a wide range of specimens has convinced the 
writer that there is still a third form on the Cruciferae which 
deserves to be accorded specific rank. The more comprehensive 
of the published descriptions have recognized a form of P. para- 
sitica with comparatively simple conidiophores which have a more 
open head. This form is quite widespread in America, and from 
the literature it appears to be common in Europe. A subspecies 
